cell boundaries. i. cell membrane a. all cells are surrounded by a thin flexible barrier known as...
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Cell Boundaries
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I. Cell Membrane• A. All cells are surrounded by a thin flexible barrier
known as the cell membrane• B. Cell membrane regulates what goes in and out of
a cell and provides protection and support.
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• C. The cell membrane is composed of a lipid bilayer layer that gives it its flexible structure. – 1. In addition the membrane is
composed of a mosaic of molecules (carbohydrates, proteins & cholesterols)
– 2. The proteins can either be peripheral located along the surface of the membrane or integral proteins which are amphipathic(have both hydrophilic & hydrophobic regions) found throughout the entire membrane bilayer
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Fluid Mosaic Model
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II. Cell Walls
• A. Cell walls are present in many organisms including plants, algae, fungi, and many prokaryotes.
• B. Most cell walls are porous enough to allow water, oxygen, carbon dioxide, and other substances to pass through easily.
• C. Cell walls are composed of thick fibers made from carbohydrates known as cellulose or chitin(fungi) or peptidoglycan(bacteria)
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III. Transport
• A. Cell transport across a membrane is dependent on 2 things– 1. Semipermeability of the membrane– 2. Size & charge of particles
• B. Plasma membrane is composed of lipids so substances that are lipid-soluble will cross easily through the only exception that also moves through is water. – Water uses proteins (aquaporins) to help regulate water flow
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IV. Diffusion
• A. Every living cell lives in a liquid environment to survive.
• B. The cytoplasm in the cell is a solution of many different substances in water– 1. Each solution in different types of cells have
distinct concentrations– Ex. 12g/3L = – 12g/6L =
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IV. Diffusion
• C. Particles of a solution are in constant motion• D. Diffusion is when particles of a solution move
from an area of high concentration to an area of low concentration; moves down conc. gradient– Think perfume bottle or a fart (random motion)
• E. When the concentration is equal on both sides the system is said to be in equilibrium– 1. Diffusion does not require the cell to use any
energy Ex. Dialysis, kidney
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Countercurrent exchange
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V. Osmosis
What if the molecule is to large to move across the membrane??
• A. The cell membrane is said to be selectively permeable meaning only certain molecules can pass; molecule that are small and have no charge can pass easily in and out of a cell.
• B. Osmosis is the diffusion of water across a selectively permeable membrane
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VI. How Osmosis Works
• A. Water will always move across a membrane from a high conc. to a low conc. (with conc. gradient) until equilibrium is reached.– 1. When two solution are equal the solutions are
isotonic to one another or “same strength”– 2. When a solution is of higher concentration to
another it is hypertonic or “above strength”– 3. A solution that is very diluted to another is
hypotonic or “below strength”
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• Based on Water potential (Ψ)
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VII. Facilitated Diffusion
• A. Molecules that are too large (lipid-insoluble) gain the help of proteins to move easily across a cell membrane this is called facilitated diffusion
• B. These channel proteins are very specific and only allow one type of molecule to move across
• C. Facilitated diffusion still does not take the use of the cells energy since it only occurs down a concentration gradient
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VIII. Active Transport
• A. Active transport moves materials in the opposite direction or against the concentration gradient– 1. Active transport requires the cell to
use energy
• B. The materials are usually moved across the membrane through the use of protein pumps– 1. Calcium, potassium, sodium ions
move across this way
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IX. Large Molecule Transport
• A. Movement of large molecules inside the cell by pinching of cell membrane into a vesicle is called endocytosis.– 1. 3 examples of endocytosis
• a. Phagocytosis – take in food• b. Pinocytosis – take in liquid• c. Receptor-mediated – particle binds to receptor (cholesterol)
• B. Movement of large molecules outside a cell is called exocytosis (neurons)
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• C. Bulk flow is one-way movement of fluids brought about by pressure.
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X. Cell Junctions
• A. When cells are in close contact they sometimes form intercellular junctions that allow cells to form strong connections, prevent passage of materials or establish rapid communication. – Types of junctions:– 1. Desmosomes – hold cells tightly together– 2. Gap Junctions – protein complex form channels allow
for communication & transfer of small molecules/ions– 3. Tight Junctions – extremely tight connection no space
between cells
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